1.1 Getting Started with Current Sense Amplifiers, Session 1: When to choose a Current Sense Amp

Hello. My name is Dan Harmon. And I'm the product marketing engineer for Texas Instruments' current sense amplifiers. In today's training. I will introduce the basic concepts of direct current sensing, which is based on Ohm's law, where the voltage across the sense element-- typically, a shunt resistor-- is measured to determine the current value. It is an invasive measurement method where power is dissipated by the sense element, the shunt resistor. I will review the strengths and weaknesses of various implementations so that you'll be able to choose the best method for your application.
Before we can look at the various circuit options, I need to introduce some basic concepts that will help you better understand those options.
The first concept is that of common mode voltage. Common mode voltage is defined, as shown in the figure, as the average voltage of the two input pins of a differential amplifier. Depending on your measurement technique and application requirements, your amplifier's common mode voltage range may be required to either handle ground or a very high voltage level.
The next concept is that of low-side or high-side measurement. A low-side implementation has the shunt resistor placed between the load and system ground. This results in the common mode voltage being essentially equal to 0 volts. However, this also disturbs the ground seen by the load, as well as preventing the ability to detect load shorts to ground.
A high-side implementation has the shunt resistor placed between the supply voltage and the load, resulting in the common mode voltage being essentially equal to the bus voltage. This allows for the system to not have any ground disturbance, as well as giving it the ability to identify ground shorts. But depending on the system voltage, may require an amplifier with a very high common mode voltage capability.
Direct current sensing typically uses a differential input amplifier. There are four main configurations that are used for current measurement. Operational amplifiers, or op amps, difference amplifiers, instrumentation amplifiers, and current sense amplifiers.
Op amps offer the most basic implementation, and typically are only used in lower-accuracy, lower-cost applications. The cost savings are offset if high-accuracy external components are used to increase the precision of the measurement. In addition, the single-ended nature of this implementation introduces additional error due to parasitic impedance between the shunt and ground.
Difference amplifiers are designed to convert large differential signals to large single-ended signals. The typical voltage drop across our shunt is small, so as not to add to the overall system load. Therefore, the difference amplifier architecture does not lend itself to most current sensing applications. The difference amplifier implementation eliminates the error due to parasitic impedance with the op amp.
Instrumentation amplifiers are a combination of a difference amplifier with a buffer on each of the inputs. This very large impedance enables the measuring of very small currents. However, the architecture limits the use to applications where the common mode voltage is within the supply voltage range.
Current sense amplifiers, also known as current shunt amplifiers or current shunt monitors, are specialized amplifiers that have a unique input stage that enables the common mode voltage to be significantly higher than the supply voltage. In addition, they integrate a high-precision, low-drift gain network that maximizes measurement accuracy. The infrastructure limits the use to applications where the current is greater than tens of microamps.
Direct current sensing is a simpler to implement and lower-cost method of current sensing than indirect magnetic methods. And current sense amplifiers offer the most comprehensive set of features to maximize current measurement performance for the widest range of applications.
For more information on TI's current sense amplifiers, please watch the remainder of the current sense video series, as well as go to www.ti.com/currentsense.
Thank you for your time today. 大家好。 我叫 Dan Harmon。 我是德州仪器 (TI) 的 电流感应放大器 产品市场工程师。 在今天的培训中， 我将介绍 直流电流采样 的基本概念， 该概念基于欧姆定律， 即通过测量 采样元件 （通常是分流电阻） 两端的电压来确定电流的值。 它是一种侵入式 测量方法， 功率将耗散在 采样元件，即 分流电阻上。 我将回顾 各种实现方法的 优缺点， 以便您能够 选择最适合 您的应用的方法。 在我们来看 各种电路选择之前， 我需要介绍 一些基本概念， 帮助您更好地 理解这些选择。 首先要介绍的 概念是共模电压。 如图所示， 共模电压的定义是 差分放大器 两个输入引脚 的平均电压。 根据您的测量 技术和应用 要求，您的放大器 共模电压的范围 可能接地，或需要达到 超高电压 水平。 第二个概念是 低侧或高侧 测量。 低侧的实现方法是 在负载和 系统接地之间 放置分流电阻。 这使 共模电压 实质上等于 0 伏。 但是，这也会干扰 负载处的地， 并影响检测 负载对地短路 的能力。 高侧的实现方式是 在电源电压 和负载之间放置 分流电阻， 使共模 电压 实质上等于 母线电压。 这使系统 没有任何地线 干扰，并 使其能够 识别对地短路。 但根据 系统电压， 可能需要放大器具有 极高的共模电压 承受能力。 通常，直流电流采样 使用的是差分输入 放大器。 主要有 4 种 用于电流 测量的 结构。 运算 放大器、 差分放大器、 仪表放大器 和电流感应放大器。 运算放大器提供最 基本的实现方式， 通常只用于 精度和成本较低的 应用。 如果使用高精度 外部元件 以增加测量 精度，则会抵消 节省的成本。 此外，由于分流器 和地之间的 寄生阻抗， 该实现方法的单端性质 会造成额外的误差。 差分放大器 用于 将较大的 差分信号 转换为较大的单端信号。 分流电阻 上的电压降很小， 从而不会增加总的 系统负载。 因此，差分 放大器的架构 并不适用于大多数 电流采样应用。 差分 放大器实现方式 能消除运算放大器 寄生阻抗带来 的误差。 仪表放大器 由差分放大器 以及在每个 输入端上的缓冲器组成。 这一非常大的阻抗 使您可以测量 非常小的电流。 但是，这一架构 只能用于 共模电压低于 电源电压范围 内的应用。 电流感应 放大器，也称为 电流分流放大器 或电流分流监控器， 是一种 具有独特的输入级， 能够使共模电压 显著 高于电源电压的专用放大器。 此外，它集成了 高精度、低漂移增益 网络，最大程度地 提高了测量精度。 该架构将其 限制为 只能用于 电流大于几十 微安的应用。 直流电 感应是一种 比间接磁传感方法 实现起来更简单且成本 更低廉的电流感应方法。 而电流感应 放大器提供了 一套最完整 的功能， 能够在更为广泛的 应用中实现最佳的 电流测量性能。 有关 TI 的电流感应 放大器的更多信息， 请观看电流感应 视频系列的其余部分， 并访问 www.ti.com/currentsense。 感谢您今天的观看。

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Date:
April 30, 2015

The Getting Started with Current Sense Amplifiers series helps engineers learn how to to maximize the performance achieved when measuring current with a current sense amplifier (also called a current shunt monitor). Session One introduces the basic concepts of direct current sensing and walks through choosing a current shunt monitor over other differential amplifier alternatives.